The present invention relates to an attrition-resistant catalyst system based on phosphorus and iron oxides, with a view to the gas phase oxidative dehydrogenation of saturated carboxylic acids to corresponding unsaturated acids, in particular with a view to the oxidative dehydrogenation of isobutyric acid to methacrylic acid in an entrained-bed reactor.
A material substance with catalytic activity for such a catalyst system has already been described in Patent FR-A-2,497,795 and is denoted by the general formula FeP.sub.x Me.sub.y O.sub.z, in which:
Me denotes at least one of the following elements: Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba, PA0 x has a value from 0.2 to 3.0, PA0 y has a value from 0.01 to 0.2, and PA0 z is the quantity of oxygen bonded to the other elements and corresponding to their oxidation state. PA0 on the one hand, plug flow in the case of the gas and the solid, permitting an accurate control of the residence time of the gaseous mass and of the solid phase, and PA0 on the other hand, it avoids the reaction zone being short-circuited by bubbles, as happens in the case of fluidized-bed reactors, PA0 possibility of reducing the water content in the reaction section formed by the upward-flow column and of regenerating the catalyst with the aid of an aqueous gas stream, especially a mixture of steam and oxygen which may also contain an inert gas such as nitrogen, as well as a reactant which compensates for any loss of phosphorus from the catalyst during the reaction cycle, PA0 and, lastly, very low water consumption in the reaction section.
Catalysts of this type take the form, inter alia, of a bulk solid. The term "bulk solid" in the context of this invention is meant to define a non-supported catalyst. As a general rule the preparation is carried out in an aqueous medium, the oxides of iron, phosphorus and of the metal Me being prepared by evaporating down an iron salt, a salt of the metal Me and orthophosphoric acid.
In accordance with Patent FR-a-2,497,795, the oxidative dehydrogenation reaction is carried out in a stationary-bed reactor. The stationary bed consists of a motionless compact charge of catalyst particles stacked in a tube. The catalyst particle size (a few millimeters in diameter) is such that it must reduce the pressure drop to a minimum but it thus involves a limitation on diffusion. To improve the yield of the desired product, the reaction is carried out in the presence of steam in a molar ratio to the substrate of approximately 1 to 74, preferably 10 to 30. Under these conditions the conversions and the selectivities are generally high, given that the flow of gas approximates plug flow and that the contact time can be controlled accurately. Thus, isobutyric acid conversion may be up to 98% and the selectivity for methacrylic acid up to 70%. Nevertheless, with an exothermic catalytic reaction, as is the case with the oxidative dehydrogenation of saturated carboxylic acids, hot spots sometimes appear, and these affect catalyst performance, especially by modifying the selectivities. It is then necessary to resort to catalyst replacement when the performance is affected. In addition, the presence of large quantities of water makes the separation of the water after reaction very costly.
It would therefore be advantageous to be able to conduct such reactions in the presence of very small quantities of water, for example in a so-called entrained-bed or transported-bed or pneumatic-transport reactor. Such reactors comprise an upward-flow column in which the gaseous reactant feed and the catalyst suspended in the latter travel concurrently upwards. The product leaving at the top of the column is separated from the catalyst in a device of the cyclone type. The catalyst is then generally sent to the top part of a fluidized-bed regeneration column in which a regenerating gas circulates; it leaves the regenerating column in the bottom part thereof, from which it is sent back to the bottom of the upward-flow column.
Such an entrained-bed reactor has the following advantages:
In addition, temperature control is efficient in the case of exothermic reactions, owing to the fact that the solid particles which are carried over, because of their high heat capacity, remove the heat of the reaction. Above all, however, the crucial advantage is the possibility of accurately separating the two stages of the catalyst operation, namely the reduction of the solid catalyst in the first step and, in a second step, its oxidation and its rehydration with a view to regeneration. oxidation and its rehydration with a view to regeneration.
A procedure in accordance with the teaching of Patent EP-A-263,005 may be followed to regenerate the catalyst.
Nevertheless, a reactor of this type imposes some constraints: thus, the catalyst must resist attrition which results from the impact of the particles on each other or on the internal walls.